The present invention relates, in general, to a compounder-type injection molding machine for mixing and then injecting materials into an injection mold.
Compounder-type injection molding machines of a type involved here are known for example from German Pat. No. DE 11 42 229, International Pat. Publication WO92/00838 or Japanese Pat. No. JP-Y36-19372. Common to all these prior art injection molding machines is the provision of a continuously operating extruder which is charged with initial plastic material, e.g., plastic granulate, to produce a plastic melt. The extruder may be configured in many different ways, e.g., as single screw extruder or twin screw extruder with screws rotating in a same direction or in opposite directions. Other configurations involve the provision of one or more feed openings for mixing several materials.
During continuous operation of the extruder screws, melt is provided constantly at the extruder outlet for transfer to an injection chamber of an injection device for subsequent introduction to an injection mold by means of a reciprocating plunger.
Compounder-type injection molding machines are useful, when several materials have to be mixed, for example, admixture of fibers to a plastic melt for providing fiber-reinforced materials.
A problem experienced in conventional compounder-type injection molding machines is the combination of a continuously operating extruder with an intermittently operating injection device. Various proposals have been made to address this problem. One approach involves the provision of a reservoir to form an intermediate storage for receiving melt delivered by the extruder while the injection phase of the injection device is carried out. As the injection device is filled again during the filling phase, the reservoir is emptied while the extruder continues to operate. Another approach involves the provision of several injection devices, which are alternatingly filled with melt. However, for a number of reasons, the various proposals are endowed with drawbacks and shortcomings. One problem of conventional injection molding machines is the absence of a constant melt quality which directly affects the material being ultimately produced. The melt quality may be adversely affected as a result of pressure fluctuations in the extruder or downstream of the extruder. Heretofore, attempts to address the problem of pressure fluctuations involve the use of dynamic pressure measurements with respect to the hydraulic pressure. These types of measurements are, however, inaccurate.
It would therefore be desirable and advantageous to provide an improved compounder-type injection molding machine to obviate prior art shortcomings and to ensure a constant melt quality.